This invention relates to RF test instruments and, more particularly, to signal level meters and leakage detectors of the type used, for example, in cable television systems. There is a need for a broad frequency range in RF test instruments, particularly those for use in the CATV industry. However, designers of such instruments must contend with a significant problem in maintaining adequate local oscillator isolation in a broadband RF front end while using low-cost printed circuit board designs and simple automated assembly processes. To prevent spurious responses, commonly known as spurs, due to harmonic mixing of the local oscillators, isolation on the order of 100 dB is required. Unfortunately, this degree of isolation is impractical at reasonable cost.
Receiver spurs occur when the first local oscillator and the second local oscillator and/or their harmonics combine in the first or second mixer and produce an output that falls within the third IF bandwidth. Traditionally, spurs are reduced to an acceptable level or eliminated by providing better isolation between the first and second local oscillators and mixers. In wideband receivers, providing this isolation leads to costly packaging, shielding, and filtering.
Many spurs in a receiver can be eliminated by switching the second local oscillator frequency for high or low side mixing as the receiver is tuned. U.S. Pat. No. 4,512,035 to Victor et al. provides one example of a receiver capable of avoiding receiver self-quieting spurious responses by selectively providing either high-side or low-side injection to the second mixer. For receivers with narrow enough IF bandwidths, this can eliminate all spurs in the third IF. In signal level meters, a large spur that falls outside the third IF bandwidth but inside the second IF bandwidth can desensitize the second or third amplifier, or the third mixer. This will cause inaccurate signal level readings for receive frequencies relatively near to the spur frequency. Switching the second local oscillator frequency alone will not eliminate this type of spur problem.
Thus, there remains a need for improvement in the elimination of receiver spurs.